1. Field of the Invention
The present invention relates generally to a servo system for a motor having a rotor and, more particularly, is directed to a servo system for servo-controlling the rotation of a motor such as a drum motor for rotating a rotary head drum of a video tape recorder, its capstan drive motor, and a motor for driving a spindle of a disk apparatus with high accuracy.
2. Description of the Prior Art
Rotating members such as a rotary head drum of a video tape recorder and a disk drive portion of a disk apparatus have to be rotated with accuracy as high as possible, and the rotational phase thereof needs to be synchronized with an external reference signal.
Therefore, a drum drive motor and a disk drive motor are servo-controlled for their rotational speed and for their rotational phase. The speed servo is effected by controlling a frequency of a rotation detecting signal FG from a rotation detecting apparatus whose frequency corresponds to a rotational speed of a motor, and hence the rotation detecting apparatus must be high in accuracy for its detection.
The phase-servo is effected in such a fashion that the phase relation between a reference position signal PG generated once per revolution of the motor and a signal REFP indicating a reference phase is controlled to have a predetermined phase relation. Generally, the phase servo is carried out by the-phase data which is available only once per revolution of the motor in order to simplify the arrangement of the phase servo loop. In this case, a very small error within one rotation of the motor is relatively small because of the inertia of the rotating member and can be ignored.
As a rotation speed detecting apparatus employed widely in the prior art, there is known a rotation detecting apparatus that comprises a rotary member and a magnetic sensor element. In this case, the rotary member which has a plurality of N and S magnetic poles magnetized at a predetermined pitch on the outer peripheral surface thereof is coaxially attached to the rotary shaft of a motor and the magnetic sensor element is located facing the outer peripheral surface of the rotary member to sense the N--S magnetic pattern.
Although the above-mentioned rotation detecting apparatus is simple in construction, it is difficult to obtain the rotation detecting apparatus which carries out the detection with high accuracy because of mechanical factors such as an error of magnetized pattern of magnetic poles of the rotary member, its eccentricity and so on. For example, even though the motor is correctly rotated at a constant speed, an error signal is generated due to the error of the magnetized pattern, causing the rotation detecting apparatus to erroneously determine that the motor is not rotated at the constant speed. Then, the servo system servo-controls the motor so as to remove the error signal such that undesired and irregular rotation is applied to the motor in order to remove the error caused by the magnetized pattern and so on. In order to solve the above-mentioned problems, there is proposed a method in which two magnetic sensor elements are located relative to the magnetized pattern at the positions displaced in phase, thereby improving the rotation detection accuracy. This previously-proposed method is disclosed in official gazette of Japanese laid-open patent No. 58-186812.
Further, the phase-servo is performed by the use of one phase data that is derived per revolution of the motor so that fine phase control within one revolution of the motor cannot be effected.
In the rotational speed control of the motor, the rotational speed of the motor is detected by measuring the cycle period of a rotation detecting signal DT having the frequency corresponding to the rotational speed of the motor and which is derived from the rotation speed detecting element coaxially attached to the rotary shaft of the motor.
This cycle measuring method has a defect that at a low speed the speed detection and hence the responsiveness in the speed control are degraded because the cycle period measuring time is increased as the rotational speed is decreased.
Therefore, in order to improve the responsiveness in the low-speed region, there is proposed a method in which, when the motor is rotated at low speed, the grade of an inclined portion of a waveform of the rotation detecting signal DT is measured. This previously-proposed method is disclosed in official gazette of Japanese laid-open patent No. 59-116050.
According to the above-mentioned method, the rotational speed of the motor can be detected on the principle that the grade of the inclined portion of the triangular wave signal generated by the rotation of the motor is proportional to the rotational speed of the motor. However, if the peak-to-peak value of the resultant triangular wave signal is not constant, the grade of the inclined portion to be detected is also changed with the result that the rotational speed cannot be accurately detected. To solve this problem, in the prior art, the gain adjustment of a triangular wave signal generator is carried out by manually adjusting its volume. In this case, however, the gain adjustment is very cumbersome and it cannot follow the changes due to temperature and aging.